The present invention relates to a scanning transmission electron microscope equipped with an energy loss spectrometer, and an electron energy loss spectroscopy using the same.
A scanning transmission electron microscope provided with an energy loss spectrometer detects each electron scattered by a specimen or transmitted through the specimen in synchronization with the scanning of an electron probe to thereby bring an intensity distribution of the scattered electron or transmitted electron into an image, and simultaneously detects an energy loss spectrum or an intensity distribution of a specific energy-selected electron in synchronization with the scanning of the electron probe by use of some of the transmitted and scattered electrons to thereby bring the intensity distribution of the energy-selected electron into image form.
A method for acquiring a scattered electron image using the conventional scanning transmission electron microscope apparatus, a method for acquiring an energy loss spectrum, and a method for acquiring an element mapping image using an energy loss spectrum have been disclosed in, for example, Japanese Patent Application Laid-Open No. 2001-266783. According to each of the methods, a scattered electron intensity is detected by an annular shaped scintillator having an aperture or opening in its center, which is used as a scattered electron detector, and is brought into image form. Then, an electron or electron beam transmitted through the opening is launched into an energy loss spectrometer, where an energy loss spectrum and an element mapping image using an energy loss spectrum are acquired.
Angular ranges of the scattered electron and the electron captured into the energy loss spectrometer are controlled by a projector lens disposed above the scattered electron detector. An image point of the projector lens corresponds to an object point of the energy loss spectrometer. The energy loss spectrometer has an object point thereof and a design value of an acceptance or capturing physical angle necessary to obtain a well resolved spectrum. The object point is set by the above methods and the setting of the physical angle corresponding to the above setting is carried out by inserting a collection angle defining aperture directly above the energy loss spectrometer. A detecting angle of the scattered electron is decided at the position of the object point of the projector lens with respect to the above condition. Positions, regions and the like for acquiring the energy loss spectrum and the element mapping image can be determined while the scattered electron image is being observed. The energy loss spectrometer forms an energy dispersion corresponding to an energy loss of the electron by use of a magnetic field prism and enlarges the focus and energy dispersion width of a spectrum by means of a multi-pole element. While the object point of the energy loss spectrometer corresponds to an image point at which the magnetic field prism is capable of forming a satisfactory dispersion, it normally has an allowable range of about 50 mm. The allowable range of the object point is decided based on an allowable range of aberration of the magnetic field prism and an allowable range in which the focus of the spectrum can be formed by the multi-pole element. Upon setting of an actual optical condition, the condition of the projector lens is determined while the S/N of a scattered electron image, the S/N of an energy loss spectrum, and an energy resolution are being measured.